Significance of Hall current and viscous dissipation in the bioconvection flow of couple-stress nanofluid with generalized Fourier and Fick laws

In the pump of different machines, the vacuum pump oil (VPO) is used as a lubricant. The heat rate transport mechanism is a significant requirement for all industries and engineering. The applications of VPO in discrete fields of industries and engineering fields are uranium enrichment, electron microscopy, radio pharmacy, ophthalmic coating, radiosurgery, production of most types of electric lamps, mass spectrometers, freeze-drying, and, etc. Therefore, in the present study, the nanoparticles are mixed up into the VPO base liquid for the augmentation of energy transportation. Further, the MHD flow of a couple stress nanoliquid with the applications of Hall current toward the rotating disk is discussed. The Darcy-Forchheimer along with porous medium is examined. The prevalence of viscous dissipation, thermal radiation, and Joule heating impacts are also considered. With the aid of Cattaneo-Christov heat-mass flux theory, the mechanism for energy and mass transport is deliberated. The idea of the motile gyrotactic microorganisms is incorporated. The existing problem is expressed as higher-order PDEs, which are then transformed into higher-order ODEs by employing the appropriate similarity transformations. For the analytical simulation of the modeled system of equations, the HAM scheme is utilized. The behavior of the flow profiles of the nanoliquid against various flow parameters has discoursed through the graphs. The outcomes from this analysis determined that the increment in a couple-stress liquid parameter reduced the fluid velocity. It is obtained that, the expansion in thermal and solutal relaxation time parameters decayed the nanofluid temperature and concentration. Further, it is examined that a higher magnetic field amplified the skin friction coefficients of the nanoliquid. Heat transport is increased through the rising of the radiation parameter.


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Uniform angular velocity µ Dynamic viscosity ρ Density σ Electrical conductivity ε 0 Thermal relaxation time parameter ε 1 Solutal relaxation time parameter σ * Stefan-Boltzmann constant θ Dimensionless temperature φ Dimensionless concentration χ Dimensionless gyrotactic microorganism η Similarity variable ε t Dimensionless thermal relaxation time parameter ε c Dimensionless solutal relaxation time parameter δ Motile difference parameter ε 2 Rotating parameter At the surface ∞ Free stream In the field of industries and manufacturing, non-Newtonian liquids (NNF) have fascinated the attention of scientists and researchers because of their multiple applications. The applications of the non-Newtonian fluids are extraction of raw petroleum, paper creation, the expulsion of plastics sheets, wire drawing, glass fiber, hot rolling, fiber technology, foodstuff, medications, material processing, oil-reservoir, nuclear and chemical industries, gemstone developments, paper creation, wire drawing, subsistence items, and others. Owing to its numerous applicability in different domains of industries and engineering, non-Newtonian fluids are discussed by different investigators and scientists. Khalil et al. 1 numerically found the non-Newtonian liquid flow over the stretchable sheet through the effect of variable liquid properties. From their conclusions, they observed that the liquid velocity becomes lower for a larger viscosity parameter. Nabwey et al. 2 have offered the effect of non-Newtonian liquid flow with motile microorganisms by using the bvp4c technique over an inclined stretching cylinder. In this evaluation, it is detected that the increment in curvature parameter, enhanced the value of absolute drag force. Sharma and Shaw 3 numerically inspected the occurrence of non-Newtonian fluid flow with the impact of nonlinear thermal radiation past over an extending sheet. They observed that the increment in the Casson liquid factor weakened the nanofluid skin friction coefficient. Khader et al. 4 scrutinized the non-Newtonian liquid flow with the employing of MHD under the effect of heat transmission by using the Chebyshev spectral method (CSM). In this examination, it can be predicted that with the higher approximation of the Hartmann number, the velocity profile of the liquid is lower. Li et al. 5 have analyzed the influence of the Lorentz forces in a flow of non-Newtonian liquid with the influence of Darcy Forchheimer by applying the convective conditions and found that the intensification in the Biot number enlarged the temperature gradient of the liquid. Dawar et al. 6 have hired the HAM approach for the analytical framework of the non-Newtonian fluid flow with a stagnation point in the attendance of the magnetic effect. In this problem, the heat transmission is enlarged due to the higher estimates of the volume fraction. Colak et al. 7 used the artificial intelligence approach to demonstrate the influence of thermal-dependent viscosity and motile gyrotactic microorganisms over the non-Newtonian Maxwell nanofluid flow. Khashi'ie et al. 8 discussed the radioactive heat transport analysis through a non-Newtonian Reiner-Philippoff liquid flow under the influence of viscosity dissipation over the shrinkable sheet with the help of the bvp4c method. In this simplification, it is noticed that the energy transportation rate is lessened through the enlargement of the magnetic parameter.
Scientists and researchers got attentive to evaluate the flow of nanoliquid for the improvement of heat transport. Nanofluids have an extensive range of implementation in several fields of industries and manufacturing such as microelectronic, solar energy, mineral oils, nuclear reactors, biomedicines, cancer therapy, thermosyphons, magnetic drags targeting, shipping, pulsating heat pipes, transformers, electronic devices, coolant in computers, lubrications in the vehicle, drug delivery, nano-medicines, fermentation science, design of heat exchangers, rubber sheets manufacturing, power generation, medical equipment's and etc. Due to these applications of nanofluid in different arenas of manufacturing and science processing, researchers and scientists have examined the nanofluid flow. Waqas et al. 9 have presented the significance of motile microbes on the nanoliquid flow over a moving sheet by using a numerical technique with the prevalence of the heat source. Muhammad et al. 10 have reviewed the nanoliquid flow by taking the convective boundary conditions with the help of the bvp4c. From their concluding points, it is perceived that the nanoliquid energy field is growing for greater values of the heat sink. Li et al. 11 arithmetically discoursed the energy transport in the nanoliquid flow under the upshot of the Hall current by using the wave frame. Ramzan et al. 12 considered a HAM method to inspect the physical consequence of the slip conditions over the mixed convection flow of nanofluid toward the exponential stretchy surface. Their concluding remarks explained that the higher estimates of nanoparticles amplified the thermal profile. Rajamani and Reddy 13 numerically demonstrated the MHD couple-stress nanoliquid flow across a permeable channel under the Joule heating effect. In this work, they used the Runge-Kutta technique for the numerically framework of their modeling. Hosseinzadeh et al. 14 have proposed the occurrence of the thermal radiation on MHD nanoliquid flow with entropy generation among the two stretchable revolving disks. In this work, it is noted that the flow stream is boosted through the augmentation of the magnetic parameter. Ramzan et al. 15 mechanically exemplified the influence of the entropic generation on the non-Newtonian liquid flow under the dipole impact through the thin needle. Algehyne et al. 16 numerically evaluated the impact of the Darcy-Forchheimer on the flow of nanofluid with bioconvective conditions through a porous vertical plate. Mishra and Manoj 17 described the thermal performance of the MHD nanofluid flow with viscosity effects and magnetic field with the use of silver Ag nanoparticles in the water base liquid toward the stretched cylinder. It is noted that heat generation and viscous dissipation have similar effects on the temperature curve. Mishra and Manoj 18 have determined the significance of the radiation, and Joule heating effect on the thermal performance of the MHD nanofluid flow. They employed the Runge-Kutta technique for the assessment of their problem mathematically. Mishra and Manoj 19 used the Buongiorno model for the numerical description of the viscous dissipation of the MHD nanofluid flow past a wedge through the porous medium. In this article, it is found that by increasing the porosity of the fluid, the rate of heat transport is enhanced. Mishra and Upreti 20 analyzed the comparative study of the Fe 3 O 4 − CoFe 2 O 4/ /EG and Ag − MgO nanoparticles in a hybrid nanofluid flow by using water as a base liquid and initiated that the thermophoresis parameter increased the rate of mass transference. Mishra and Manoj 21 have discussed the physical significance of the silver Ag-water and copper Cu-water nanofluid model with the existence of viscous dissipation and thermal transport. It has been obtained that heat transport is increased for suction/injection parameters. Mishra and Manoj 22 have investigated the viscous dissipation, and suction effects on the nanofluid flow toward the Riga plate. Giri et al. 23 proposed the generalized Fourier and Fick's Law for the computation of the heat-mass transference mechanism on the Casson nanoliquid flow with a magnetic field toward the stretched surface. Das et al. 24 have evaluated the comparative mixture of the aluminum oxide Al 2 O 3 and Graphene nanoparticles on the flow of hybrid nanofluid with Hall effect above the slender stretchy surface. In this analysis, they obtained that, for the wall thickness parameter, the concentration profiles show dual behavior. Giri et al. 25 have offered the bioconvection flow of the nanoliquid with the Stefan blowing effect toward the permeable surface. Giri et al. 26 have demonstrated the Darcy-Forchheimer porous medium to analyzed the influence of chemical reaction on the mixed convection flow of nanofluid and decrement role of nanofluid Sherwood number is examined for higher Darcy-Forchheimer parameter. Further studies on the nanofluid flow problems are discussed in [27][28][29][30] .
The magnetohydrodynamic (MHD) flow problems attained the curiosity of the investigators and scientists because of their useful implementations in various industrial and engineering areas. In thermal nuclear reactors, www.nature.com/scientificreports/ MHD is used to control the diffusion rate of neutrons. Some more applications of the MHD flow problems are crystal formation, chemical reactions, MHD generators, metallurgical processes, blood flow measurements, liquid metals, aerodynamics, geothermal engineering, designs of nuclear reactors, petroleum industries, geophysics, flow meter, crystal growth and etc. Therefore, the scientists and investigators discuss the MHD flow problem in their area of research. Azam 31 has utilized the concepts of energy flux theory for the calculation of the energy conveyance on the MHD flow of Maxwell nanofluid flow toward the moving surface. In this problem, growing values of the Maxwell parameter lessened the fluid speed. Mahabaleshwar et al. 32 have assessed the nanoliquid flow with SWCNTs and MWCNTs nanoparticles over the shrinkable sheet in the incidence of the magnetic effect. In this review, it is noted that the energy profile is enhanced for the higher suction effect. Ramzan et al. 33 have discoursed the Darcy-Forchheimer impact on the viscoelastic fluid flow with motile gyrotactic microorganisms by using the stretched surface. They obtained that by growing the Prandtl number, the temperature profile is lower. Iqbal et al. 34 evaluated the MHD flow under the existence of the buoyancy effect through a vertical stretchy sheet. They determined that the nanofluid thermal profile is amplified due to the intensifying estimates of the magnetic factor. Rasheed et al. 35 analytically demonstrated the viscosity and joule heating influence on the Casson fluid flow with a magnetic field over the porous moving surface. In this evaluation, it is accomplished that energy curve is intensifies with a strengthening of the Prandtl effect. Ramzan et al. 36 have surveyed the MHD flow of nanoliquid with autocatalysis chemical reaction and Hall current over the rotating disk by using the HAM technique.
From the last few decades, the concepts of Hall current played a dynamic role in various science and engineering processes. The usage of Hall current in distinct area of industries and engineering field are power generators, planetary hydrodynamic, turbines, refrigerators loops, magnetometers, pumps, electric modifiers, Hall sensors, Hall accelerator pedals, and so forth. Therefore, a lot of researchers have discussed the Hall effect in their study. Elattar et al. 37 have discoursed the development of the Hall current on hybrid nanofluid above the stretchable surface by using the bvp4c technique and obtaining that the Hall parameter raised the fluid particles speed. Khan et al. 38 analytically debated the MHD flow of liquid with Hall current over the stretchable surface under the mixed convective boundary conditions, it is noted that the augmentation in Prandtl number augmented the energy outline. Abbasi et al. 39 computed the impact of the Hall current during the estimation of peristaltic nanoliquid flow with entropic characteristics and solved their model with the aid of the homotopy perturbation scheme and attained that the energy curve is amplified with the augmentation of the Hartmann number. Rana et al. 40 have discoursed the theory of energy flux with the existence of the Hall current through the revolving disk. Ramzan et al. 41 have measured the role of ferrofluid with Hall current toward the bidirectional stretching surface by exploiting the theory of Cattaneo-Christov.
In fluid mechanics, bioconvection is an attractive phenomenon and has a vast array of implementations in the field of life science technology, biomedical, and bioscience. The process of bioconvection occurs, when mixed nanofluids are subjected to mass and energy transmission. The study of motile gyrotactic microbes become an interesting phenomenon for scientists and investigators due to its enormous range of applications in countless areas of manufacturing and industries. Yin et al. 42 have estimated the behavior of the motile microorganisms on the MHD Sisko flow over an extending cylinder along with bioconvective boundary conditions and thermal radiation and discoursed that the expansion in curvature parameter led to a decay in the velocity profile. Gangadhar et al. 43 calculated the joule heating upshot on the bioconvection Oldroyd-B nanoliquid flow through a vertical sheet under the consequence of the motile microorganisms. In this simulation, it is distinguished that for larger estimates of the thermophoresis constraint, the density motile microorganism profile is reduced. Muhammad et al. 10 analyzed the importance of the motile microorganism on the Jeffery nanofluid flow above the stretched sheet. They noted that the greater estimates of the thermophoresis factor enlarged the nanofluid energy curve. Famakinwa et al. 44 numerically illustrated the motile microbes on the MHD nanoliquid flow. They examined that the solutal graph of the nanoliquid is lesser for the intensifying value of exothermic reaction. Nazeer et al. 45 numerically calculated the chemical reaction effect on the bioconvective nanoliquid flow under convective conditions with the help of the R-K method through a Riga plate. Waqas et al. 46 have deliberated the MHD non-Newtonian nanofluid flow along an extending sheet. In this modeling, they also take the effects of thermal diffusion and conduction. Puneeth et al. 47 have discoursed the model for heat-mass transport of Casson nanoliquid flow due to the movement of the motile microorganisms and Brownian motion. Khan et al. 48 numerically determined the effect of motile microorganisms through the hybrid nanoliquid flow over the revolving disk with slip conditions.
In the presence of the above studying literature, the present study explored the mechanism of the vacuum pump oil (VPO) base fluid and Fe 3 O 4 (Iron oxide) nanoparticles on the flow of the couple-stress nanoliquid with Hall current and Darcy-Forchheimer past a spinning disk. Vacuum pump oil (VPO) has a lot of engineering and industrial applications such as lubrication processes, cooling, sealing, noise reduction, corrosion protection, and flushing containments, small air compressors, hydraulic circulating lubrication systems, and vacuum pumps. Due to these applications of Vacuum pump oil (VPO), the present study is analyzed with the mixing of Fe 3 O 4 (Iron oxide) nanoparticles into the Vacuum pump (VPO) base fluid. For the exploration of the mass and energy transport characteristics, the theory of the Cattaneo-Christov mass and energy flux is employed. The computation for viscous dissipation, and Joule heating effect is accomplished. To incorporate the bioconvection phenomena, the idea of the motile gyrotactic microbe is utilized. Heat can be transferred through different ways but in the existing study, the role of the thermal radiation is computed on the existing couple stress nanofluid problem, therefore, the main aim of thermal radiation in the present work is to transfer the heat by the electromagnetic waves due to the rotational and vibrational movement of the fluid particles. That's why the present study has a lot of applications on the basis of thermal radiation such as coals, hot bricks, flames, etc. Simulation of the existing problem is accomplished by operating the HAM technique. Variations in velocities, temperature, concentration www.nature.com/scientificreports/ and gyrotactic microorganism are computed against pertinent flow parameters. At the end of this study, we will be able to answer the following questions: • What is impact of Fourier's and Fick's laws on the exiting couple stress nanofluid problem?
• How does the heat transport behave against thermal radiation effect?
• How does the motion of the liquid particles affect by a Lorentz force with the applications of magnetic field?
• What is the behavior of nanofluid velocity and temperature when Iron oxide nanoparticles is mixed up into the vacuum pump oil base fluid? • What is effect of motile gyrotactic microorganism on the couple stress nanofluid? • Why we preferred homotopy analysis method for the simulation of the existing problem?

Problem formulation
Let us assume the steady, incompressible flow of couple-stress nanofluids with Hall current past a rotating disk. The disk is rotating or moving through the porous medium by obeying the Darcy-Forchheimer theory. The features of the Cattaneo-Christov heat and mass flux model are discussed for the simulation of heat and mass transmission. By adding the Fe 3 O 4 (Iron oxide) nanoparticles to the base liquid, the couple stress nanofluid is formed. VPO is taken to be the base liquid. The bioconvection mechanism is deliberated under the motile gyrotactic microorganism. Due to the stretching as well as the rotating of the disk the motion of the fluid is produced. About the z-axis the disc is rotating with a uniform angular velocity . In the directions of (r, ϕ, z) , the velocity terms are expressed by u , v and w . The magnetic effect B 0 is normally employed to the gyrating disc. At the surface of the disk the temperature is T w , the concentration is C w and motile gyrotactic microorganism is n w . But far from the rotating disk the temperature is T ∞ , concentration is C ∞ and the motile microorganism is n ∞ . The physical illustration of the present study is discussed in Fig. 1.
The leading equations for the current flow problem are determined by taking into account the assumptions on the flow behavior described above [49][50][51][52][53][54] : The velocity factors are expressed by u , v and w along the directions of x, y, z-axes, the nanofluid dynamic viscosity µ nf , ρ nf is the density of the nanoliquid, σ nf is the nanofluid electrical conductivity, B 0 is the magnetic field strength, the Hall current is m , is the inertia coefficient, the thermal relaxation time parameter is ε 0 , the Brownian diffusion coefficient is D B , ε 1 is the solutal relaxation time parameter, b is used for chemotaxis constant, and the Brownian diffusion coefficient of microorganism is D m . Nanofluid properties. The thermophysical properties of the nanofluid are: The thermophysical characteristics of the VPO − Fe 3 O 4 nanoliquid are summarized in Table 1. Similarity transformations: With the applying of similarity transformation from Eq. (9), we get: After the simplification the non-dimensional constraints are discussed here. The porosity parameter is symbolized by is the couple stress nanofluid parameter, the Darcy-Forchheimer factor is Fr , the thermal relaxation time parameter is represented by ε t = ε 0 � , the Schmidt number is specified by Sc = ν f D B , ε c = ε 1 � is the solutal relaxation time parameter, the bioconvection Lewis number is Lb = ν f D m , the bioconvection Peclet number is Pe = bW c D m , δ = n ∞ n w −n ∞ is the motile gyrotactic microbe difference parameter and the rotation parameter is ε 2 = c � . In current study, mathematically the physical quantities of interest such as skin friction coefficients, Nusselt number, Sherwood number and motile density number are defined: The non-dimensional form of the C f , C g , Nu , Sh and Nn are obtained after applying the similarity transformations on the Eq. (16):

Solution of the problem
In fluid dynamics, the flow problem exists in the form of higher-order nonlinear differential equations and it is not easy to solved these higher-order nonlinear differential equations, therefore different mathematicians and researchers have devolved a different analytical and numerical techniques. Also, by using the homotopy analysis method, different scientists and mathematicians have discussed their problems. The advantages of the homotopy analysis method over the other methods are: 1 Pr www.nature.com/scientificreports/ • Presented technique is independent of large and small constrains.
• Homotopy analysis method is used for both strongly and weakly nonlinear problems.
• With the applying of the homotopy analysis method, any systems of nonlinear PDEs are solved without linearization and discretization. • With the utilization of the homotopy analysis method, the series and convergent solution of the system is obtained. • It is a linear scheme and does not require any base function.
Keeping in mind the above-mentioned advantages of the homotopy analysis method, the existing modeled mathematical framework is solved with the implementation of the HAM technique. So, analytical solutions of the highly nonlinear ODEs (10)(11)(12)(13)(14) with relevant boundary conditions (15) are obtained by manipulating the HAM method. with where ℓ i (i = 1 − 11) are the arbitrary constants.
Zeroth-order deformation problems. The zero-order deformation are discussed as here the embedding parameter is ℜ , and the non-zero auxiliary parameters are h F , h G , h θ , h φ and h χ . N F , N G , N θ , N φ and N χ are the nonlinear operator and discussed as: According to Taylor's series expansion, we have:

Result and discussion
The physical description of the steady flow of couple stress nanoliquid across a rotating disk in a porous media is elaborated. Simulation for the present problem is made on the basis of the homotopy analysis method. The fluctuation in different flow profiles of the nanofluid are discussed against discrete flow parameters. Also, the behavior of distinct flow constraints on the physical quantities of interest is determined.
Comparison of the present study. Figures 2 and 3 are addressees for the comparison of the present work with the Hayat et al. 51 work. From Fig. 2, the velocity profile of the present work shows good agreement with the velocity profile of Hayat et al. 51 work by assuming Fr = 0, k 2 = 0, and k 1 = 0. Further in Fig. 3, it is noticed that the temperature profile shows good agreement with the temperature profile of Hayat et al. 51 Pr (71) χ m (η) = χ * m (η) + ϒ 10 exp(η) + ϒ 11 exp(−η). www.nature.com/scientificreports/ a retardation force is produced. So, the fluid velocity is slow down due to the porosity of the surface because Darcy-Forchheimer and porosity of the surface are associated with each other. Figure 5 explains the outcome of k 1 on the velocity profile of the nanofluid in x−direction. The reducing performance of the velocity profile of the nanofluid in x−direction is examined for higher k 1 . Figure 6 uncovered the conduct of the velocity outline in x−direction versus greater values of couple stress fluid parameter k 2 . In this graph, the reduction in velocity curve in x−direction is examined with the enrichment of the couple stress fluid parameter k 2 . Figure 7 is made to      www.nature.com/scientificreports/ velocity in x−direction is diminishing with the expanding of m . The role of φ on the velocity outline in x−direction is scrutinized in Fig. 9. In this investigation, the decreasing behavior in the velocity curve in x−direction is discoursed for escalating values of φ . Physics of the nanoparticle volume fraction explain that the density of the nanofluid is enhances when the nanoparticle volume fraction is increases. An intermolecular force between the       Figure 10 illustrates that how nanofluid velocity is affected by the Darcy-Forchheimer constraint Fr in y−direction. In this enquiry, it is noted that the velocity curve in y−direction is decreases for maximum values of Fr . The factor Fr has a nonlinear relation against the fluid flow. Further, as the value of Fr is rises, in the motion of fluid particles a retardation force is produced. So, the fluid velocity is slow down due to the porosity of the surface because Darcy-Forchheimer and porosity of the surface are associated with each other. In Fig. 11, the impact of the k 1 on the velocity graph in y−direction is discussed. In this Figure, it is seen that the velocity outline in y−direction is amplified when the porosity factor k 1 is enhanced. Figure 12 displays the fluctuation of velocity outline in y−direction versus discrete values of k 2 . The velocity of the nanofluid is amplifies due to the larger value of couple stress parameter k 2 in y−direction. Figure 13 explains the evaluation of velocity outline in y−direction for larger values of M . It can predict that the increment in magnetic parameter produced a Lorentz force in the liquid motion. Therefore, the velocity profile is decays with the increase of M . Figure 14 determined the velocity curve in y−direction versus m . In this graph, it is detected that the flow speed is decreased due to the increment of m in y−direction. Figure 15 examined the effect of the nanoparticles volume fraction φ on the fluid velocity in y−direction. In this evaluation, it is noted that the nanofluid velocity profile in y−direction is diminishing with respect to larger values of φ . Physics of the nanoparticle volume fraction explain that the density of the nanofluid is enhances when the nanoparticle volume fraction is increases. An intermolecular force between the particles of the fluid becomes stronger due to this mechanism, that's why the velocity of the nanofluid is diminutions with the intensification of the nanoparticle volume fraction.
Temperature profile. The behavior of the Eckert number Ec , thermal relaxation time parameter ε t , magnetic field parameter M , radiation parameter Rd and nanoparticle volume fraction φ on the nanofluid temperature profile are discoursed in Figs. 16, 17, 18, 19 and 20. Figure 16 examines the role of the Ec on nanofluid temperature profile. According to this observation, it is detected that the energy curve is enhances via larger values of Ec . From the physics of the Eckert number, it is observed that the internal energy of the nanofluid is changed into kinetic energy with the increase of Ec . The temperature and kinetic energy are directly proportional to each  www.nature.com/scientificreports/ other. Therefore, by increasing Ec , both kinetic energy and nanofluid temperature are increases. Figure 17 discussed the deviation of the nanoliquid temperature for distinct values of ε t . In this inquiry, it is found that when ε t is increases then the nanofluid temperature is decreases. By increasing the thermal relaxation time parameter ε t , the particles of the material require more time to transfer heat to its adjacent particles. Further, it is observed that heat transfer quickly throughout objects at ε t = 0. So, at ε t = 0, the nanoliquid temperature is higher. Fig-Figure 16.    Figure 19 represents the behavior of energy outline versus the rising values of Rd . From the definition of the thermal radiation, it is observed that thermal radiation is also known as the electromagnetic radiation and is generated due to the thermal motion of particles in a matter. Further, it is examined that with the increase of the thermal radiation temperature of the nanofluid is also increases. The relationship   www.nature.com/scientificreports/ between the radiative heat flux and Rosseland radiative absorptivity explains that the radiative heat flux increases due to the declines of the Rosseland radiative absorptivity as a result the radiative heat transport is enhances which consequently increase the temperature of the nanofluid. Further, due to the thermal boundary layer thickness the temperature of the nanofluid is improves. Moreover, if Rd = 0, the effect of radiation is negligible but in the case of Rd → ∞ , then the radiation effect become more significant. It is identified that intensifying values of Rd boosted the energy contour. The influence of the φ on the energy is established in Fig. 20. It prominent that the nanoliquid energy is higher for growing values of the nanoparticle volume fraction φ. The ratio between the momentum and mass diffusivity is known as the Schmidt number. The mass diffusivity of the nanofluid is decreases due to the increase of the Schmidt number. So, the concetartion profile is lower for higher Schmidt number. The effect of solutal relaxation time parameter ε c on mass curve is discussed in Fig. 22. In this inspection, it is noticed that the concentration of the nanoliquid is decreases for higher values of solutal relaxation time parameter ε c . At higher ε c , the liquid particles require more time to diffuse. Through the whole material, the fluid particles disperse quickly at ε c = 0. So, the concentration of the nanofluid is dominant at ε c = 0.
Gyrotactic microorganism profile. Figures 23,24 and 25 explained the effects of the temperature difference factor δ , bioconvection Lewis number Lb and Peclet number Pe on the nanofluid motile gyrotactic microorganism profile. Figure 23 depicted the effect of δ on the nanofluid gyrotactic microorganism outline. In this enquiry, it is analyzed that the nanoliquid gyrotactic microorganism profile is surges with the surging values of the temperature difference constraint δ . Figure 24 determine the behavior of Lb on the nanoliquid gyrotactic microorganism profile. It concluded that the nanofluid gyrotactic microorganism profile is declines when the bioconvection Lewis number Lb is increases. Figure 25 elucidates the upshot of the Pe on nanofluid gyrotactic         Fig. 32. In this Figure, it is detected that the Nu of the nanofluid is amplifies with the rising of Rd . From the definition of the thermal radiation Rd , it is observed that thermal radiation is also known as the electromagnetic radiation and is generated due to the thermal motion of particles in a matter. Further, it is examined that with the increase of the thermal radiation temperature of the nanofluid is also increases. The relationship between the radiative heat flux and Rosseland radiative absorptivity explains that the radiative heat flux increases due to the declines of the Rosseland radiative absorptivity as a result the radiative heat transport is enhances which consequently increase the temperature of the nanofluid. Further, due to the thermal boundary layer thickness the temperature of the nanofluid is improves. Moreover, if Rd = 0, the effect of radiation is negligible but in the case of Rd → ∞ , then the radiation effect become more significant. Hence it is clear that with the increase of the thermal radiation both temperature and Nusselt number are increases. So, higher is thermal radiation as a result higher is the wall temperature.

Conclusion
The physical significance of the vacuum pump oil (VPO) base fluid and Fe 3 O 4 (Iron oxide) nanoparticles on the flow behavior of the couple stress nanofluid with Hall current and Darcy-Forchheimer theory along the spinning disk in a porous medium has been investigated analytically. In the present flow analysis, energy transport phenomenon is established under the idea of Cattaneo-Christov heat and mass flux approach. Simulation for viscous dissipation, thermal radiation and Joule heating effect are considered. Bioconvection mechanism is discussed by incorporating the impression of motile gyrotactic microorganism. The HAM method is utilized for the solution of the existing study. Influence of several flow parameters on the different profiles is computed. Key findings of the existing study are mentioned as: • The higher Darcy-Forchheimer parameter, porosity parameter, couple stress fluid parameter, magnetic parameter, nanoparticles declined the velocity profile in x-direction. • Increment in Darcy-Forchheimer parameter, magnetic parameter, Hall current and nanoparticles led to increase the velocity profile in y−direction. • An augmentation behavior of velocity in y−direction is observed for porosity factor and couple stress fluid.
• The nanofluid temperature is lesser for Eckert number while the magnetic field and radiation factor amplified the nanofluid temperature. • Greater the Schmidt number lower the concentration profile.
• The augmentation in the temperature difference parameter and Peclet number has increased the motile microbe profile of the nanoliquid. • The decrement performance in the motile microorganism profile is noticed for bioconvection Lewis number.
• Skin friction coefficients C f and C g are lower due to higher, power index number and Darcy-Forchheimer parameter. • Higher magnetic field increased the skin friction coefficients C f and C g . • Nusselt number Nu is higher against radiation parameter.